Street lighting reflector with parabolic upward curvature formation



, 1952 P. H. MITCHELL 2,619,581

STREET LIGHTING REFLECTOR WITH PARABOLIC UPWARD CURVATURE FQRMATIONFiled 001;. 22, 1948 2 SHEETS--SHEET l [rm/enter PelCiVa/ Elf/'12: 4e

NOV. 25, 1952 P MlTcHELL v 2,619,581

STREET LIGHTING REFLECTOR WITH PARABOLIC UPWARD CURVATURE FORMATIONFiled Oct. 22, 1948 2 SHEETS-SHEET 2 [rm en for Perc/va/ H Ni/c/re/lPatented Nov. 25, 1952 STREET LIGHTING REFLECTOR WITH PARABOLIC UPWARDCURVATURE FORMATION Percival H. Mitchell, Toronto, Ontario, CanadaApplication October 22, 1948, Serial No. 56,010 13 Claims. (01. 24025)This invention relates to improvements in reflectors for use withincandescent lamps and particularly to annular reflectors with avertical axis, for reflecting and concentrating light rays from flatring filament lamps centrally located within the reflector.

The principal object of the invention is to providea novel reflectorform which will, even when of very limited size, systematically obtainfrom a horizontal flat ring filament a maximum concentration ofreflected light directed at relatively high angles in a vertical plane,whereby light with such concentrated intensity at high angles isdirected downward to be incident adjacent the verges of the circularhorizontal area illuminated by the filament to augment the lightintensity at vertical axis whereby, from successive points upward on thereflector, the light emanating from substantial lengths on thecircumference of a flat ring filament as light source, is reflected anddirected in a beam centered on the vertical axis and within narrowangular boundaries in the vertical plane to obtain a maximum intensityof reflected light at the desired critical downward an.- gle ofconcentration.

This invention is applicable to inside mirrored lamps and to reflectorsof metal or mirrored glass exterior to an incandescent lamp bulb. As ineach application the refraction by the glass bulb of the incandescentlamp more or less affects the direction of the critical reflected raysanother featureis' a simple means of compensation for such refractionadaptable to the system of determining the reflector curvatures.

Referring to the drawings,

Figure 1 shows an annular reflector with the scheme of development ofthe reflecting zones.

FigureZ is a diagram showing filament images as reflected by thereflectors in Figure 1.

Figure 3 shows an annular reflector of the type in Figure '1 external toa conventional incandescent lamp.

Figure 4 shows an incandescent lamp with inside mirrored surface as areflector of the type in Figure 1. I I

Annular reflectors, in combination with incandescent lamps, are widelyused for street lighting to give illumination on a circular area and infurther combination with refractors or secondary reflectors the lightrays may be concen trated within the circular area to effect asym-'metric distribution with relatively higher intensities on roadway areas.Incandescent lamp filaments have various forms and reflected lightdirected to the illuminated area is an overlapping of luminous images ofthe filament reflected frominnumerable reflecting points composing thereflector surface. When the filament is in the form of a flat ring lyingin a plane normal to the lamp axis and centered on the axis the filamentimages, as reflected by reflecting points on the reflector surface, areeither straight lines, when the reflecting point is in the plane of thefilament, or ovals, from other reflecting points, above or below thisplane, the ovals having relatively short minor axes. In conventionalmethods of describing the concentrating upward curvature of thereflectors for use with such lamps the centrev of the flat ringfilament, at the intersection of the filament plane and the verticaloptical axis, is taken as the focus for the curvature, which isgenerally parabolic. This has been acceptable when relatively largereflectors are used, such as with a diameter of ten times the filamentcircle diameter but for small reflectors, with diameters, say, of fivetimes the filament circle diameter, it is notable that the lightintensity obtainable by concentrating reflectors is relatively low. Bythe methods I employ the intensity obtainable by concentratingreflectors is substantially increased.

Flat ring filaments are standard construction for general servicemultiple lamps up to 200- watt size. The filaments are not completecircles but may be four-fifths of a circle, the open space being betweenthe supporting conducting wires leading to the filament. The filament inplan may not be circular but may be four sides of a pentagon. Forpurposes of description'and design, in the following, the filament isconsidered as a complete circle and the effect of divergence from thisshape is shown later. For any oval aspect of the filament as viewed froma point on the reflector the luminous intensity of any short length offilament is considered as directly proportionate to the wholecircumferential length; this is acceptable as a general rule.

In street lighting luminaires for use with multiple incandescent lampsup to 200 watts size it is desirable to have small reflectors so thatthe whole device may be of relatively low cost. As suitable for such useI show reflectors nominally five inches in diameter. The same system ofdesign may be applied to any size of reflector.

A conventional beam angle for concentration for street lighting purposesis at '75 degrees above the nadir or 15. degrees'below a horizontalplane and correspondingly the cut-off of the reflector bottom is at 80degrees above the nadir which is measured at 10 degrees downward fromthe centre of the filament. The reflectors shown are based on theseangles but other angles can be used as desired. While the best resultsare obtained at lamp settings for the desired angle of beam, by raisingor lowering the lamp relatively to the reflector the beam will belowered or raised and within a few degrees of shift the results aregenerally satisfactory.

In general, the light distribution of a street lighting luminaire, in avertical plane, includes direct light from the lamp source from thenadir up to the cut-off and reflected light from the re flector,incident from the light source, from the cut-off up to the top of thereflector, the reflector top limit being controlled by the necessity ofclearance of the neck of the lamp bulb. The lower part of the reflectorcontributes light to the beam and the upper part contributes light atangles from under the beam to part-way down to the nadir. Theimprovements shown are concerned with the lower zones of the reflectorcontributing light to the beam and at angles immediately above and belowthe beam angle.

Figure 1 shows the reflector curvature required for a specific conditionof .a fiat ring filament in a horizontal focal plane and centered on avertical optical axis with a filament ring diameter of one inch and thenominal diameter of the reflector bottom is five inches. The desiredreflected ray concentration is maximum concentration over a one-degreezone between 74 /2 and 75 degrees above .the nadir; that is, theconcentration desired is for a Iii-degree beam. The concentration overone degree is an arbitrary measure. In this figure refraction through alamp bulb is not considered but as, in general, the rays will bediverted from their incident paths when passing through the glass bulb,means for compensating for this effect will be shown.

In Figure 1 the curved line extending from M up to S is composed of thejoined parabolic arcs MN, NOP and PQR and a circular or elliptical arcRS. This curved line rotated about the vertical optical axis YYdescribes the annular reflector. ML is the reflector bottom and thediameter at ML is, in the example shown, 5 inches. XX is a focal planenormal to YY which intersects at C. A and B on the focal plane representthe two points on a flat ring incandescent filament as cut by the planeof the figure. The diameter of the flat ring filament in the example is1 inch so that A is inch from C. The line MC is inclined degrees aboveML intersecting YY at C, locating the focal plane and making aconventional SO-degree cut-off. Through A three lines ee', if and go aredrawn inclined at 15 /2, 15 and Bi degrees respectively, to XX; theseare parabolic axes for the parabolic arcs MN, NOP and PQR respectively,with the point A as focus common to all these arcs.

Commencing with the fixed point M, MN is a parabolic arc with as as axisand A as focus; this continues to N at which point lines N to A and N toB make the angle ANB equal to one degree. The parabolic arc NOP has ffas axis and A as focus, NA being a parabolic radius from A for bothadjacent arcs MN and NOP with their parabolic axes at 15 /2 and 15degrees to XX respectively. As viewed from N the whole ring filamentappears as a much flattened oval with a minor axis equivalent to onedegree and a major axis equivalent of about 24 degrees length, measuredangularly. This parabolic arc NOP intersects the focal plane at O and asviewed from O the ring filament appears as a straight line. At

. P the angle between lines PA and PB is again,

one degree. The parabolic arc PQR has gg' as axis, inclined 14 degreesto XX and A as focus. The terminus of this are R; is determined by aline Ra drawn from R inclined at 14 degrees to XX so that-it passesunder the reflector bottom at L on the opposite side with a clearance ofthe order of inch. The curved arc RS may be elliptical with one focus atA and a remote focus on Ra in the vicinity of and under L or may be anequivalent circular arc; it is only in the vicinity of R on this arethat reflected rays contribute to the light concentration in thecritical one de gree. The parabolic radius AN is common to the parabolicarcs MN and NOP but the parabolic axes and focal lengths differ;similarly the parabolic radius AP is also common to the adjacentparabolic arcs and the complete curvature of the joined arcs iscontinuously smooth. Q is a point on the reflector where lines from Q toA and C may be the angle AQC 1 /2 degrees; this is not constructionalbut is for reference in Figure2.

The lines Ma, Mb, Na, Nb etc. show paths of reflection of points A and Bfrom reflecting points M, N etc. on the reflector, respectively, Ma andRa being inclined at 15 and 14 degrees respeotively to XX.

Figure 2 is a diagrammatic view showing ring filament images (M), (N),(O), (P), (Q) and (R) as reflected from points M, N, O, P, Q and Brespectively, on the reflector in 'Figure 1. These images are to scaleangularly, that is, the

minor axes of the ovals represent or correspond to the angles betweenthe lines from each respective reflecting point to A and B, and. themajor axes represent or correspond to the diameter of the filament ringin the focal plane as viewed from the respective reflecting point. Thepoints on the major axes corresponding to C in Figure 1 are indicated(C). The corresponding locations of A and B in each image, shown as (A)and (B), are at the termini of the minor axis and of C, shown as (C), atthe intersection of the major and minor axes. The minor axes arecoincident with the trace of the vertical optical axis (Y) (Y).

In the diagrams two datum lines TT and UU, located by reference to A,are at 7 1 and 75 /2 degrees respectively, and are shown to the sameangular scale. It will be appreciated that since the light from Aincident at R strikes a parabolic reflecting surface having A as focusand line gg' passing through A and inclined at 75 degrees to the nadiras axis, it will be reflected at 75 degrees. 1 Therefore datum line UUcoincides with (A) in Figure 2. The portion of the filament between UUand TT contributes to a beam between 75 /2 and 74 degrees. The length ofthe filament image between these two lines relatively to the wholecircumference of the oval thus is taken as the measure of theconcentration effected. For comparison with the possible concentrationusing the point C in Figure 1 as the focusand a line through C inclined15 degrees to XX as axis for describing the parabolic arc correspondingto ME in Figure l, the datum lines W and WW are drawn one-half degreeabove and below the location of (C) in each oval. Itwill be noted thatdatum lines are com.- mon to each system of reflector in (N), (O) and(P) and, in these, concentrations are equal. For oval images (M), (Q)and (R) the concentration when A is the focus is measured by the longarc of the side of the oval image and when C is the focus by the twoshort arcs at the ends of the oval image. For (M) the relativeconcentrations are of the order of 7 to 1; for (Q), 10 to 1; for (R), 8to 1. As measured from A in Figure 1 the angular length of are from M toR is 32 degrees while the angular length of arc from N to P, in whichthe concentration is equal for both systems is approximately degrees.Outside this 5-degree anglethe concentration by using A as focus isgreater than by using the. conventional focus at C.

It will be noted in these diagrams in Figure 2 that when the minor axisof image ovals is one degree or less in angular length the wholefilament image is centered on the focus in line with the verticaloptical axis YY. When the minor axis of image ovals is greater than onedegree in angular length the critical portions of the filament lengthare similarly centered when my reflector is used, but if the focus C isused the twov critical portions of filament, at the ends of the ovals,are remote from the vertical optical system axis YY each being locatedup to half the major axis or some 12 degrees angularly away from thisaxis. In annular distribution this remoteness is of no consequence butwhen auxiliary devices such as secondary reflectors or refractors areused to obtain asymmetric distribution, centered on asymmetric axes, forexample up and down a street, these widely spaced concentrations offilament light cannot satisfactorily contribute to the desired axialasymmetric concentration.

In Figure 2 it will be noted that while in (O) the straight line imageand the ovals in (N) and (P) are well centered on a 75-degree beam axisthe ovals below as represented in (M) with the lower terminus of theminor axis at 74 degrees have the major portion of the ovals above 75degrees indicating that reflected light rays are directed upward toseveral degrees above 75 degrees. This is a desirable feature and isconsistent with accepted practice. It is possible however to reversethese ovals from M to N so that the top of the ovals is at 75 degreesand.

all the reflected rays are directed at and below 75% degrees. In Figure'1 the point N on the reflector was located so that rays reflected fromA and B would be directed at 74 /2 and 75 /2 degrees respectively. N isa point on a parabolic arc with axis inclined at 74 degrees to the nadirthrough A. Then N is also a point on a hypothetical parabolic arc with Bas focus and axis through E inclined at 75 /2 degrees. The are MN can bemade with B as focus with the parabolic axis hh through B inclined: at75 /2 degrees so 6 that reflecting point between M and N on this arcreflect rays from B at 75 /2 degrees and from A at lower angles. Thereflector arcs above N are made with A as focus and the arcs join to becontinuously smooth. The effect of this can be shown in Figure 2 bylocating the datum lines 8's and T'T at the top of the oval image (M) asfrom point ,M. Thus all reflected light from the reflector is at andbelow 75 /2 degrees with the same intensity in the critical one degreebut with increased in-' tensity below this. There is then no reflectedlight above 75% degrees.

For any reflecting point between M and R, in Figure 1, with A as focus,the same point is on a hypothetical parabolic arc with B as focus andwith a parabolic axis through B inclined to correspond with the angle ofthe reflected ray path of B as source. Then at any point between M and Rthe focus can be shifted from A to B, or vice versa, with a consequentvariation in the scheme of concentration and vertical distribution. Theactual amount of light incident on the zone from M toR, from thefilament, is fixed in quantity. The rearrangement of this quantity oflight between fixed angular boundaries and immediately.

outside these boundaries can effect a substantial range of distributionsfor practical results in street or area lighting.

Figures shows the reflector and filament as in Figure 1 but with a glassfilament enclosing lamp bulb K, 3% inches in diameter, whichis standardfor ZOO-wattgeneral service lamps; such bulbs have the centre for theherispherical bottom at F, of an inch below the focal plane XX'. Thevertical axis is Y'Y' with intersection with XX at C. A and B show thefilament as cut by the plane of the figure. Three reflecting points M, Oand R are shown corresponding to M, O and R in Figure 1. The reflectedray paths of A, M'a', Oa' and Ra meet the glass bulb at m, o and 1. Onpassing through two curved glass Walls of the bulb to emerge from thebulb these rays will be refracted and the angular relation of theemerging paths to the incident paths may be calculated, or measured bysimple laboratory means. The deviations may be of the order of 80minutes and 40 minutes downward for rays from M and 0' respectively and12 minutes upward for rays from R. A simple correction for thesemagnitudes of refraction is to locate a focus to be used instead of A.This focus may be located by drawing M'H inclined 80 minutes above MAand OI 40 minutes above O'A and the intersection of these two lineslocates D; a line from R. to D is acceptably close to 12 minutes belowRA. Using D as focus instead of A and with similar procedure indevelopment of the series of parabolic arcs the paths of rays reflectedfrom MO and with A as source, will be raised and the path from R loweredso that on emergence from the bulb these and intermediate paths will besubstantially correct.

In Figure 3 the point Z locates the bottom edge of a 7-inch diameterreflector with SO-degree cut-off and rays from A reflected from areflect ing point at Z will meet the glass bulb at z. This indicates apractical dimensional limitation for small reflectors with maximumdirectional control for this lamp bulb size as light rays from acorresponding point on a larger reflector meeting the glass bulb will becloser to the bottom of the bulb and cannot, satisfactorily, havecompensation for refraction. It is well known that the bulb bottom,beyond this region, is in efiect M and pointsintermediate a "blind zonedue to externalv and. internalv refiection and extremes of refraction.

Figure 4-shows a reflecting surface as in Figure 1 adapted to an insidemirrored incandescent lamp. The lamp L has a conventional externalmaximum diameter of 5 inches. The vertical axis is Y"Y" and the focalplane is X"X" with intersection at C". The mirrored interior surface ofthe glass enclosing bulb extends from M" located by a line drawn from C"inclined degrees below- 1:x". A and 3" represent thev one-inch diameterring filament lying in the focal plane, as cut by the plane of thefigure; this is centered on C". The lower portion of the glass bulb isshown, arbitrarily, as hemispherical with centre at E on Y"Y" where aline ZEZ drawn parallel with X"X" is normal to the curve above M"extended downward. When the curved surface from M" to R" is developed asin Figure 1, light rays reflected from a luminous point at A" will bedirected from M" along Ma inclined at degrees to X"X"; from 0" along0"a" inclined at 15 degrees to XX"; and from R" along Rfr" inclined at14 degrees to X"X". These path lines meet the inner bulb surface at m, oand 1", respectively, and on passing through the curved glass arerefracted, those through the glass at m and 0 being lowered and those atr being raised. The angle of change is slight, in this case, and may beof the order of minutes for those at m and r and only 3 minutes for raysat 0-. A substantial correction maybe made by drawing a line from M"inclined at an angle of 20 minutes above M"A intersecting X"X" at D thenD is used as the common focus for the several parabolic arcs forming thecurve from M" to R" with construction of the whole curvature from M toS" as described for Figure 1. Then from M" the ray M"A-" is reflected tobe 20 minutes high, from O" the ray will be, as before, 3 minutes lowand from R" will be 20 minutes low and on emission from the bulb will besubstantially at the desired angle. Intermediate points on the curvaturewill reflect light rays from A" with intermediate angles of correctionand there will be a systematic compensation for refraction through thebulb. For other practical forms of bulbs, including composite forms ofthe sealed beam type, corresponding correction means can be employed.

In this case an alternative can be used. If A is used as focus and theWhole reflector curvature from i to R. is one parabolic arc with theparabolic axis through A" and inclined 15 degrees to X"X" light raysfrom A" will be reflected at M" to be at 75 degrees but by refractionwill be lowered approximately 20 minutes below l5 degrees; at O" thereflected rays will be lowered 3 minutes; and at R" the reflected rayswill be raised, by refraction, 20 minutes. These rays then establish azone of concentration with limits of 20 minutes above and below '75degrees.

Figure 1 shows the system of reflector development using three parabolicarcs for the maximum concentration of light rays within one degreecentered on a 75-degree axis. An alternative system is to use oneparabolic are extending from M to R in Figure 1 with focus at A and withaxis 1? inclined at 15 degrees through A. While for reflecting points inthe region of the focal plane the concentration will be equal for bothsystems, at other reflecting points the reflected filament imagescontribute only to the one-half degreev above. or below. the.75edeg1tee. axis; and.

thus. the concentration: within one .degree will be less. concentrationbut concentration substantially greater than obtainable by using C .asthe focus.;: In Figure 2 inv each oval it is obvious thattthe midwaypoint on the minor axis between the .two

boundaries. of the critical one-degree, that is themidway point betweenUU. and TTisat or .corresponds to 75 degrees inclination, then for eachoval a point on the focal plane, near A..but .to-..

ward C, in Figure 1 could have been. used. at.

the respective focus with a 75-.degree axis. In (R) in Figure 2thispoint would .be approximately one-seventh of the distance from A .toC;

in v(Q), onethird; in (N) (O) and (P) it .could. be acceptably at A, Cor any point between; in.

(M) it would be at one-quarter the distancesfrom A to C. A compromisecommon focus would. be located one-quarter the distance from. A to C, onthe focal plane with M N O P Q R as one parabolic arc with axis at 75degrees through.

the focus. Such a compromise would give sub stantial concentrations froma. reflecting zone extending from M to beyond Q and beyond this towardsB there would be a greater lengthtof luminous filament contributing tothe critical one degree than if the focus had been at C. As a furtheralternative compromise the one parabolic focus could be located midwaybetween A. and C and over the whole parabolic arc there would be agreater concentrating effect than if C had been used. As a practicallimitation in these alternatives the alternative focus would be closerto A than to C. 7

As stated, filaments of the incandescent lamps used in combination withthese reflectors are not complete circles due to the gap between theconducting wires. When the open space, that is without a luminousfilament, occurs in the position corresponding to the focus A intheFi'gures 1, 3 and 5, there is a deficiency in the intensity ofreflected light in the critical one degree but generally with goodintensity at other high angles. When the open space occurs in theposition of B, in the same figures, there is good intensity in thecritical one degree but a deficiency at other high angles. If A is thefocus for arcs of the reflector surface above N,in Figure l, and B isthe focus for the arc MN and theopen space in the filament occurs at Aor B the deficiency in the critical one degree. is not so great. When inthe horizontal distribution the deficiency is at right anglestov thestreet axis, in

street lighting applications, the effect is asym metric, which may befavourable. If the filament is oriented within the luminaire this effectcan be used advantageously.

Most of the Well made commercial lamps have filaments reasonably closelyconforming in plan to arcs of a circle so that the above analyses holdessentially true. When adherence to true circles is justified, one ormore extra filament supports can be incorporated'into the lampconstruction. When the filament conforms to four or five tangents to acircle there is, in effect, a systematic variation in beam intensityaccording to the different aspects of the filament from respectivereflecting points on the reflector.

In the range of use of reflectors of this type, in certain applicationsthe complete 360 degrees of annular reflector of the form described maynot be required and only those sectors contributing to .asymmetric beamsmay be of these forms While adjacent or intervening sectors may Therecan be compromises .tov obtainless.

'be of other curvatures adapted to other distributions favourable topavement illumination in areas outside the asymmetric beam paths. It isto be understood that while descriptions refer to annular reflectors ofcontinuous form, reflectors incorporating sectors of limited annularextent having the described curvatures and as elements of reflectors ofcomposite form are included within the scope of my invention.

What I claim as my invention is:

1. An annular reflector surface having a vertical axis and a focal planein right angular relation thereto, a flat ring-type filament lightsource centered on the vertical axis and lying in said focal plane, saidreflector surface extending below said filament and having a smoothupward curvature in any plane through said vertical axis, the lowerportion of said upward curvature being formed of three joined lengths ofparabolic arcs having foci lying adjacent the circumference of said ringfilament, and the axis of each parabolic are being inclined angularly tothe axis of the adjacent parabolic arc and'at an angle to the nadirgreater than 65 degrees and extending in a direction of desired lightconcentration with the axes of the first and third parabolic arcsextended forming an acute angle therebetween determining the angulardispersion of reflected light concentrations to provide a lightconcentration whereby from successive points upward on the reflector thelight emanating from substantial lengths on the circumference of saidring light source is reflected and directed in a beam centered on thevertical axis and within the narrow angular boundaries corresponding tothe angle between the axes of the first and third parabolic arcs toobtain maximum intensity of reflected light at the desired downwardangle of concentration. r I

2 A reflector as claimed in claim 1 in which the foci of said parabolicarcs are common and lie on the circumference of said ring filament.

3. A reflector as claimed in claim 1 in which the first of saidparabolic arcs has its focus lying on the circumference of saidring'filament at a point on the remote side of said vertical axis andthe third of said parabolic arcs has its focus on said ring filament ata point on the adjacent side of said vertical axis. v

4; The combination with a flat ring type incandescent filament, of anannular upwardly curved reflector adapted to concentrate light in a highangle beam having an angular dispersion of the order of one degree, saidreflector having a vertical axis onwhich said filament is centered and afocal plane above the lower edge thereof -onwhich said filament lies,said reflector being generated by the revolution about the vertical axisof a smoothly curved line the lower part of which is defined by aplurality of parabolic contiguous arcs, said arcs having a common focus'mediate arc symmetrical above and below: the

focal plane and of a length limited toits ter- -mini being'point areasreflecting light from points adjacentthenear and the remote side of-thefllament to be coincident with the boundary 10 angles whereby fromsuccessive points upward on the reflector the light emanating fromsubstantial lengths on the circumference of said ring light source isreflected and directed in a beam centered on the vertical axis and"within narrow angular boundaries corresponding to the boundaries definedby the axes of said upper and lower parabolic arcs to obtain a maximumintensityof reflected light at the desired downward angle-ofconcentration.

5. A reflector as claimed in claim 4, in which said filament is enclosedin a glass bulb and the focus of said parabolic arcs is locatedintermediate the filament circumference and ..the vertical reflectoraxis to direct rays reflected from said reflector at an angle to emergeafter refraction by .the curved glass of said bulb Within. the desiredangular boundaries. '6. An annular reflector having a vertical'axis anda focal plane in right angular relation thereto, a flat incandescentring filament centered on said axis and laying in said focal plane, saidreflector having a smooth upwardcurvature in any plane through itsvertical axis with the lower portion of said upward curvature beingdefined by a plurality of contiguous arcs, comprising a parabolic areextending either sideof said focal plane having a focus adjacent thecircumference of said filament and a parabolic axis lying in said planethrough the vertical axis and inclined at an angle below the focal planesubstantially less thana quarter right angle equal to the desired angleof maximum light concentration, a parabolic are extending below saidfirst-mentioned parabolic arc and having a focus adjacent thecircumference of said filament and a parabolic axis lying in'said planethrough the vertical axis and inclined at an angle of the order of adegree to the aforesaid parabolic axis to determine a lower angularboundary of maximum light concentration, and a parabolic are extendingabove said firstmentioned parabolic arc and having a focus adjacent thecircumference of said filament and ,an axis lying in said plane throughthe vertical axis and inclined at a vertical angle of the order of adegree to said first-mentioned parabolic axis to determine an upperangular boundary of maximum. light concentration. a 7. A device asclaimed in claim 6 in which the end points of said first-mentionedparabolic arc define with points on said filament on opposite sides ofsaid vertical axis an angle corresponding to the angle betweentheboundaries of desire maximum light concentration. 4

8. An annular reflector surface for a ring type filament having avertical axis and a focal plane in right angular relation thereto inwhich a ring filament is adapted to be located centered on said axis,said reflect-or surface extending below-said focal plane and having asmooth, upward curvature in any plane through said vertical axis, thelower portion of said upward curvature being constituted by three joinedlengths of parabolic arcs each having its focus lying adjacent thecircumference of said ring filament, the intermediate parabolic arehaving an axis inclined downwardly at an angle of the order of 15 to thefocal plane and the parabolic arcs above and below said intermediate arehaving axes inclined downwardly the one at an angle of the order of 15/2" and the other at an angle of the order of 14 to said focal'plane. 9.An annular reflector for a ring type filament having a reflect-orsurface generated by the rotation of a composite curve about the axis ofsym- 'metry of the filament, the generating curve of saidsurfaceincludingasection to formthe lower portion of the reflectorsurface made up of three joined parabolic arc lengths each having afocus approximately coincident with the circumference of said filament,the axis ofithe intermed'iate'of said parabolic arcs being inclineddownwardly at an angle of the order of 15 toythepl-ane of said filament,the axes of the other parabolic arcs above and below said intermediateare ,being inclined downwardly to the plane of said. filament, theoneatv an angle of the order of Li and the other at an angle .of the orderof 15%; to said filament pl-ane.

10. An annular reflectorfor a ring type filament having azrefiectorsurface generated by the rotation of a composite curve about the axis ofbolic .arc lengths each having a focus approximately coincident .withthe circumferenc .of said fllamenttheaxis of the intermediate of saidparaboliclarcs being inclined downwardly at an angle of the order of 15to the plane of said filament, the axis .of the other parabolic .arcsabove and below said intermediate are being inclined downwardlyto theplane .of said filament, the one atan angle of Li and the other at .anangle of theorder of 15%;" to said filament plane, and

said elliptical arc having a remote focus lying within the divergence of,thenaxes of the first :and third parabolic arc lengths.

11. An annular reflector surface for a ring type filament having ;avertical axisland a focal plane in right angular relation thereto inwhichea ring filament is adapted .to be located centered on said axis,:saidreflector surface extending .below the focal plane and beingdescribed by the revolution of acurve about the vertical axis, saidcurve including a parabolic arc length centered on the focal plane and aparabolic arc length aboveland below said first arc length, saidarclengths all having a focus approximately coincident with thecircumference ofsaid filament and adjacent arc lengths having commonparabolic radii to form=a continuous curvature but having differentfocal lengths, the first of said arcs having a parabolic axisintersecting said vertical axis and inclined below the focal plane at anangle substantially less than a quarter rightangle, the arc lengthseither side of said first arclength having parabolic axes'inclinedto'the aforesaid parabolic axis :at a small-acute angle and definingboundariesof light dispersion of a beam re- -fiectedfrom the reflectorsurface portion described by'said parabolic-arc lengths directed on theparabolic axis of said first parabolic arc length centered on-saidfocal'plane.

12.-Anannular street lightingbeam producing device, comprising anannular reflector arranged coaxially with a glass enclosed flat ringincandescent lamp filament and extending below and above the-focal planein which the filament lies and having a reflecting surface described bythe-revolution of a curve about the vertical axis, said curve includinga parabolic arc length centered-on the focal'plane and a parabolicarclength above and below said first arc length, said are lengths-allhaving a focus approximately -coincident with the circumference of saidfilament, and adjacent arc lengths havingcommonparabolic radii to form acontinuous curvature but having different focal lengths, the first ofsaid arcs having a parabolic axis intersecting said vertical axis andinclined below the focal plane at an anglesubstantially less thana-quarter right angle, the arclengths either side of said first arclength having parabolic axes inclinedto the aforesaid parabolic axis ata small acute angle, said surface projecting a light beam with-a zoneof'maximum intensity of light concentration along said parabolic axis ofsaid first-mentioned parabolic arc at a-high angle above the nadir andbelowa horizontal plane, saidbeam having the zone ofmaximumconcentration of projected light contained between specific angularl-y;spaced zonal boundaries defined :by the parabolic axes of said parabolicarc lengths-above and below said first-mentioned arc length, thereflected images .of the luminous filament from zones of the reflectorlying in the focal plane and-immediately. adjacent being projected -tobe within the-said zonal boundaries, and reflected oval images of theluminous filament from reflecting points in said surfaces in zones belowthe immediate zone at the focal plane being projected-to have the loweredge of each image coincident with the lower zonal boundary andthew-holeimage lying partly Within the zonal boundaries and partly abovethe zonal boundaries, and reflected oval images :of the luminousfilament from reflecting points on said surface lying within a limitedzone above th immediate zone at the focal plane being projected to havethe upper edge of each image coincident with the upperzonal boundary andthe whole image lying partly within the zonal boundaries and partlyabove.

13. An annular reflector having .a verticalaxis and ,a focal plane inright angular relation to the axis thereof, a flat ring-type filamentlight source centered ,on the vertical axis and, lying in said focalplane, said reflector having a smooth upward curvature in any planethrough its vertical axis such that reflector points on the reflectorlying in the focal plane reflecting images of the said filamentasstraight-line imagesand reflecting points-on the reflector above andbelow the focal plane reflecting images of the said filamentas flattenedovals widening in their upward diameter measurable along their minoraxes as the reflecting points recede from the focal plane,

saidsurface being described by the revolution about the vertical ,axisvof-a curve having at least the'lower portion COmDriSedby apluralityofcontiguousparabolic arcs each are having its focus 10D thecircumference'of; said flat ring type filament light sourceandihavingits axis inclined angularly to the axis-of the adjacentparabolicarc,-said arcs comprising an intermediate arc with its parabolic axisinclinedqto the nadir at an angle greater than andzcorresponding to thedesired angle of downward concentration'of light rays and substantiallysymmetrical with and extending above and below the focal plane to arespective zone in which a-reflectingpoint reflectsoval images of thesaid filamentlight source to have a diameter on its minor axis toprovide an annular spread of substantially less than 10 ofthelzoneofeffectedconcentration of light rays centered on the parabolic axis ofsaid intermediate arc, and the axes of the adjacent parabolic arcs beinginclined to the axis of the said intermediate arcto effect fromsuccessive points downward on the lower adj acentpa-rabolic. arc asuper-REFERENCES CITED The following references are of record in the file ofthis patent:

Number Number 14 UNITED STATES PATENTS Name Date Mygatt Nov. 5, 1912Benford Feb. 21, 1928 Beck et a1 Oct. 23, 1934 Halvorson Mar. 1, 1938FOREIGN PATENTS Country Date Great Britain May 14, 1931 Germany June 27,1935

